1. Field of the Invention
The present invention relates to a piezoelectric transformer converter, and particularly to a switching power supply for low-voltage high-current electric power.
2. Description of the Prior Art
Various piezoelectric transformer converters have been proposed, of which several examples follow. U.S. Pat. No. 3,790,826, for example, discloses a piezoelectric transformer converter using longitudinal vibration mode, and Japanese Patent Laid-open No. 54686/86 discloses a self-excited piezoelectric transformer converter. Both of these disclosed piezoelectric transformer converters are for generating high voltage, and the use for which they are intended is therefore entirely different from that of the present invention. In addition, the latter piezoelectric transformer converter is provided with an additional electrode for self-excited oscillation.
U.S. Pat. No. 5,118,982 further discloses a piezoelectric transformer for power use using thickness-extensional vibration mode, but this U.S. patent discloses only a piezoelectric transformer and makes no mention of any peripheral circuits such as a switching circuit, a rectification-smoothing circuit, an oscillation circuit, or an output control circuit.
The present inventor has proposed in Japanese Utility Model Laid-open No. 54492/92 an invention in which a piezoelectric transformer using thickness-extensional vibration mode is supplemented with a switching circuit and a rectification-smoothing circuit, but in this case, no mention is made of self-excited oscillation or output control.
In Japanese Patent Laid-open No. 49846/92 and Japanese Patent Laid-open No. 133657/92, the present inventor has proposed a piezoelectric transformer converter that uses frequency characteristics to effect output-voltage control over piezoelectric transformer gain by separately excited oscillation, but these cases do not involve the self-excited oscillation system.
Although the present inventor has also proposed a piezoelectric transformer converter that uses self-excited oscillation in Japanese Patent Application No. 55020/92, this proposed piezoelectric transformer converter uses an additional electrode on the surface of the thickness-extensional vibration mode piezoelectric transformer. Moreover, this proposed invention is silent regarding output control.
As stated above, in a system for controlling switching frequency by separately excited oscillation (Japanese Patent Laid-open No. 49846/92 and Japanese Patent Laid-open No. 133657/92), the gain frequency characteristic of the piezoelectric transformer has an extremely high Q, and in addition, because this frequency characteristic varies greatly depending on the manufacturing process and temperature, actual setting of the frequency is difficult. Accordingly, self-excited oscillation is desirable, but when employing an additional electrode as in conventional cases, manufacture is complex.